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The accusations mark a significant and alarming escalation in the increasingly belligerent rhetoric being directed at Britain.

While senior officials have publicly expressed their fury at the British media for its coverage of the case, the Kremlin has stopped short of including the government itself in its attacks.

But after Britain demanded the extradition of another former KGB agent, Andrei Lugovoi, to stand trial for Mr Litvinenko's murder, the Kremlin appears to be shifting its strategy.

Yesterday Mr Lugovoi held a press conference in which he alleged that MI6 had been involved in the murder. Kremlin critics suggested that Russian government or intelligence officials had written Mr Lugovoi's statement in an attempt to deflect suspicion being directed at Moscow.

Mr Litvinenko, who died in November three weeks after being poisoned with the radioactive substance Polonium-210, was an outspoken critic of President Vladimir Putin and was regarded as a traitor by many in the FSB, the KGB's main successor.

State controlled media has sought to pin the blame on Boris Berezovsky, a once powerful tycoon living in London, suggesting that he carried out the murder to besmirch the reputation of Mr Putin, his great enemy.

But with British police apparently unwilling to swallow that theory, analysts say the Kremlin is now trying to show that Britain has no interest in solving the murder — either because MI6 was involved or because London is itself trying to smear Mr Putin.